Programmable alternating current (ac) load having regenerative and dissipative modes

ABSTRACT

A programmable AC load in communication with an equipment under test (EUT) is disclosed. The EUT generates an equipment under test voltage. The programmable alternating current (AC) load includes an active load profiler (ALP), a grid-connecter inverter, and an operational mode selector. The operational mode selector is in communication with an AC side of the grid-connected inverter. The operational mode selector places the programmable AC load in either a regenerative mode where the equipment under test voltage is sent to a main grid or a dissipative mode where the equipment under test voltage is dissipated by heat.

FIELD

The disclosed system relates to a programmable alternating current (AC)load and, more particularly, to a programmable AC load for producing awide variety of load profiles that operates in either a regenerative ordissipative mode.

BACKGROUND

Aircraft electrical systems usually include electrical components,devices, and equipment that require certain power qualitycharacteristics. Thus, the aviation industry requires a programmablealternating current (AC) load that creates a variety of load profilefunctions. The programmable AC load may include combinations of passivecomponents such as dissipative resistors, inductors, capacitors ormotors. By changing or re-arranging the combination of these components,a limited number of load profiles may be generated. Some types ofprogrammable AC loads also include semiconductors such as diodes andinsulated-gate bipolar transistors (IGBTs).

Several drawbacks exist with the programmable AC loads currentlyavailable. For example, some programmable AC loads may only be capableof producing a limited number of load profiles. In particular, passivecomponents and relatively slow power semiconductors may not be suitablefor achieving some types of load profiles such as steady-statefundamental component profiles, harmonic load profiles, transient loadprofiles, modulation load profiles, and combinations thereof. Inparticular, diode bridge rectifiers may be used to generate harmonicload profiles. However, the harmonic patterns that are generated usingthe diode bridge rectifiers are very limited. Also, the passivecomponents within the programmable AC load need to be adjusted in orderto vary the magnitude and phase of the harmonic. Moreover, motor loadsare typically used to achieve low frequency current modulation profiles.In this case, the modulation pattern is limited in terms of programmablecurrent modulation magnitude and frequency as well as combining themodulation load profile with other types of load profiles, such astransient load profiles or harmonic load profiles.

In addition to the above-mentioned issues, many programmable AC loadscurrently available are purely dissipative. In other words, the energyprocessed by the programmable AC load to test a power source may bedissipated by heat, which is not cost-effective and unnecessarily wasteselectrical energy. This issue may be further compounded if theprogrammable AC load is used in a test having a relatively longduration. Thus, there exists a need in the art for a more efficient,cost-effective programmable AC load that generates a wider variety ofload profiles.

SUMMARY

In one aspect, a programmable alternating current (AC) load incommunication with an equipment under test (EUT) is disclosed. The EUTgenerates an equipment under test voltage. The programmable AC loadincludes an active load profiler (ALP), a grid-connecter inverter, andan operational mode selector. The ALP creates a specific load profilesent to the EUT. The ALP includes a voltage source inverter having an ACside and a direct current (DC) side, where the AC side of the voltagesource inverter in communication with the EUT for receiving theequipment under test voltage. The grid-connected inverter has an AC sideand a DC side. The DC side of the grid-connected inverter is incommunication with the DC side of the voltage source inverter of theALP. The grid-connected inverter provides a regulated DC voltage sourceto the voltage source inverter of the ALP. The operational mode selectoris in communication with the AC side of the grid-connected inverter. Theoperational mode selector places the programmable AC load in either aregenerative mode where the equipment under test voltage is sent to themain grid or a dissipative mode where the equipment under test voltageis dissipated by heat.

In another aspect, a programmable alternating current (AC) load incommunication with an equipment under test (EUT) is disclosed. The EUTgenerates an equipment under test voltage. The programmable AC loadincludes an active load profiler (ALP), a grid-connecter inverter, andan operational mode selector. The ALP creates a specific load profilesent to the EUT. The ALP includes a voltage source inverter, at leastone plug-in high frequency module, and a control module. The voltagesource inverter has an AC side and a direct current (DC) side. The ACside of the voltage source inverter is in communication with the EUT forreceiving the equipment under test voltage. The plug-in high frequencymodule is connected in parallel with the voltage source inverter andgenerates higher order harmonic load profiles. The control module is inoperative communication with a corresponding plug-in high frequencymodule. The control module sends a control duty signal to thecorresponding plug-in high frequency module indicating a switchingfrequency. The grid-connected inverter has an AC side and a DC side. TheDC side of the grid-connected inverter is in communication with the DCside of the voltage source inverter of the ALP. The grid-connectedinverter provides a regulated DC voltage source to the voltage sourceinverter of the ALP. The operational mode selector is in communicationwith the AC side of the grid-connected inverter. The operational modeselector places the programmable AC load in either a regenerative modewhere the equipment under test voltage is sent to the main grid or adissipative mode where the equipment under test voltage is dissipated byheat.

In yet another aspect, a programmable alternating current (AC) load incommunication with an equipment under test (EUT) is disclosed. The EUTgenerates an equipment under test voltage. The programmable AC loadincludes an active load profiler (ALP), a grid-connecter inverter, andan operational mode selector. The ALP creates a specific load profilesent to the EUT. The ALP includes a voltage source inverter having an ACside and a direct current (DC) side, where the AC side of the voltagesource inverter is in communication with the EUT for receiving theequipment under test voltage. The ALP also includes a control module inoperative communication with the voltage source inverter. The controlmodule sends a control duty signal to the voltage source inverterindicating a switching frequency and a duty cycle. The grid-connectedinverter has an AC side and a DC side. The DC side of the grid-connectedinverter is in communication with the DC side of the voltage sourceinverter of the ALP. The grid-connected inverter provides a regulated DCvoltage source to the voltage source inverter of the ALP. Theoperational mode selector is in communication with the AC side of thegrid-connected inverter. The operational mode selector places theprogrammable AC load in either a regenerative mode where the equipmentunder test voltage is sent to the main grid or a dissipative mode wherethe equipment under test voltage is dissipated by heat.

In still another aspect, a method of creating a specific load profile bya programmable alternating current (AC) load that is sent to anequipment under test (EUT) is disclosed. The EUT generates an equipmentunder test voltage. The method includes creating a specific load profilesent to the EUT by an active load profiler (ALP). The ALP includes avoltage source inverter. The method also includes providing a regulateddirect current (DC) voltage source to the voltage source inverter by agrid-connected inverter. Finally, the method includes placing theprogrammable AC load in either a regenerative mode where the equipmentunder test voltage is sent to the main grid, or a dissipative mode wherethe equipment under test voltage is dissipated by heat by an operationalmode selector in communication with the grid-connected inverter.

Other objects and advantages of the disclosed method and system will beapparent from the following description, the accompanying drawings andthe appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the disclosed testing system includinga programmable alternating current (AC) load in communication withequipment under test (EUT), the programmable load including an activeload profiler (ALP) and a grid-connected inverter;

FIGS. 2A-2C are exemplary illustrations of inverters that may be usedfor the ALP and the grid-connected inverter shown in FIG. 1, where FIG.2A is a single phase, 2-level inverter, FIG. 2B is a three-phase 2-levelinverter, and FIG. 2C is a three-phase cascade 3-level inverter;

FIG. 3 is a schematic diagram of an alternative embodiment of theprogrammable AC load, where a parallel interleaving topology includestwo or more substantially identical ALPs connected in parallel with theEUT shown in FIG. 1;

FIG. 4A is a schematic diagram of a programmable AC load based on hybrid5-level voltage source inverter topology;

FIG. 4B is a schematic diagram of one of the bi-directional DC-DCconverters shown in FIG. 4A;

FIG. 5 is a schematic diagram of an alternative embodiment of theprogrammable AC load, where the ALP includes one or more plug-in highfrequency modules;

FIG. 6 is a schematic diagram of a control structure for one of theplug-in high frequency modules shown in FIG. 5, where the control systemincludes a signal synthesis unit and a control module;

FIG. 7 is a block diagram of the signal synthesis unit shown in FIG. 6;

FIG. 8 is a block diagram of an error block shown in FIG. 6 that is partof the signal synthesis block; and

FIG. 9 is a schematic diagram of an another embodiment of the ALP shownin FIG. 1, where the ALP includes control structure for currentmodulation.

DETAILED DESCRIPTION

As shown in FIG. 1, FIG. 1 is a schematic view of an exemplary testingsystem 10 including a programmable alternating current (AC) load 20 incommunication with equipment under test (EUT) 22. As used herein, theterm AC load refers to an electrical circuit that draws current fromanother circuit where the drawn current by the AC load is at least oneof continuously variable, periodically variable, non-repeatingtransient, or temporarily steady-state. Similarly, the term directcurrent (DC) load refers to an electrical circuit that draws currentfrom another circuit where the drawn current by the DC load is nearlyconstant and non-variable. A DC load could have some moderate drift orchange in bias over time, but is generally characterized in that it doesnot change. In this manner, the AC load described herein is generallycharacterized in that it is variable, but it can have at least some DC,non-variable attributes in a narrow time sample.

The EUT 22 may be any type of electrical machine that generates AC powersuch as, for example, a synchronous generator operating at 400 Hertz.The EUT 22 may be subject to testing by the programmable AC load 20. Asseen in FIG. 1, the programmable AC load 20 may include an active loadprofiler (ALP) 30, a grid-connected inverter 32, and an operational modeselector 40. The operational mode selector 40 may be used to connect theprogrammable AC load 20 to either a main grid or utility line 42 duringa regenerative mode, or to one or more dissipative elements 44 in adissipative mode. In the non-limiting embodiment as shown in FIG. 1, theutility line 42 may include a utility or mains frequency of about 60Hertz, however those skilled in the art will appreciate that thisillustration is merely exemplary in nature. For example, in alternativeembodiment, the mains frequency may be about 50 Hertz.

As explained in greater detail below, the ALP 30 of the programmable ACload 20 may be used to provide a variety of load profile functions tothe EUT 20. In particular, the ALP 30 may emulate actual conditions thatthe EUT 22 typically experiences while operating an aircraft. Forexample, the programmable AC load 20 may generate steady-state loadprofiles, lower harmonic load profiles (the exact harmonic orders dependon the specific topology of the system), transient load profiles,modulation load profiles, higher order harmonic load profiles, andcurrent modulation load profiles.

The ALP 30 may include an inductor L_(alp), a voltage source inverter(VSI) 50, and a capacitor C (the resistance r_(alp) illustrated in FIG.1 represents parasitic resistance of the inductor L_(alp)). An AC side52 of the VSI 50 is in operative communication with the EUT 22, wherethe inductor L_(alp) is located between the VSI 50 and the EUT 22.Specifically, the EUT 22 may generate an equipment under test voltageV_(EUT). The equipment under test voltage V_(EUT) may be sent to the VSI50 via a current line 46. The ALP 30 may also include a DC side 60. TheDC side 60 of the VSI 50 is connected to a DC side 62 of thegrid-connected inverter 32. The grid-connected inverter 32 provides aregulated DC voltage source V_(DC) to the VSI 50. The ALP 30 may createone or more load profile functions that is ultimately provided to theEUT 22 based on the regulated DC voltage source V_(DC) from thegrid-connected inverter 32.

The VSI 50 as well as the grid-connected inverter 32 may be any type ofinverter for converting DC voltage sources into AC voltage sources in acontrolled manner. For example, referring to FIGS. 2A-2C, the VSI 50 orthe grid-connected inverter 32 may be a single phase, 2-level inverteras seen in FIG. 2A, a three-phase 2-level inverter as seen in FIG. 2B,or a three-phase cascade 3-level inverter as seen in FIG. 2C. As seen ineach of FIGS. 2A-2C, the exemplary inverters may include a switching legstructure, where a plurality of power transistors 54 in combination withantiparallel diodes 56 are located on each leg 58 of the inverter. Theswitching elements 54 may be, for example, insulated-gate bipolartransistors (IGBTs) or metal-oxide-semiconductor field-effecttransistors (MOSFETs). In the embodiment as seen in FIG. 2B, theinverter is a three-phase inverter having a neutral leg configuration.FIG. 2C is an illustration of a multilevel inverter that is capable ofproducing more than two voltage levels.

Those skilled in the art will readily appreciate that the illustrationsshown in FIGS. 2A-2C are merely exemplary in nature, and that the VSI 50and the grid-connected inverter 32 shown in FIG. 1 may include a varietyof configurations. In particular, it is to be understood that thetopology of the VSI 50 determines the topology of the remainingelectrical components within the testing system 10. For example, in theembodiment as shown in FIG. 1, if the VSI 50 is a 2-level inverter, thenthe grid-connected inverter 32 may also be a 2-level inverter as well.Moreover, a DC link 70 including a positive rail and a negative rail maybe used to connect the DC side 60 of the 2-level ALP 30 to the DC side62 of the 2-level grid-connected inverter 32. Thus, the topology of theVSI 50 determines the topology of both the grid-connected inverter 32and the DC link 70. Those skilled in the art will also readilyappreciate that if the VSI 50 is a multilevel inverter (i.e., more thantwo voltage levels), then a multilevel DC link (not illustrated inFIG. 1) may be used to connect the ALP 30 to multiple DC voltage sources(not illustrated). FIG. 4A is an illustration of the VSI 50 as amultilevel inverter connected to multiple isolated DC-DC converters, andis described in greater detail below.

Continuing to refer to FIG. 1, an AC side 66 of the grid-connectedinverter 32 may be connected to the operational mode selector 40 througha line filtering inductor L_(gci), where r_(gci) represents theparasitic resistance of the inductor L_(gci). A control module 76 is inoperational communication with the operational mode selector 40, a firstuser input 78, and a second user input 79. The first user input 78 andthe second user input 79 may be any type of device configured to receiveuser-generated input from an individual and send a control signal to thecontrol module 76 indicative of the input generated by the individual.For example, in one embodiment, the first and second user interfaces 78,79 may be a keypad or touchscreen.

The individual may determine whether the testing system 10 shouldoperate in the regenerative mode or the dissipative mode based onvarious parameters of the testing system 10. For example, the individualmay check to see if the testing system 10 is properly connected to theutility line 42 first before selecting the regenerative mode. In theevent the testing system 10 is not connected to the utility line 42 orif there is no utility line 42 currently available, then the individualmay select dissipative mode. Once the individual makes thedetermination, he or she may manipulate the first user input 78accordingly.

Once a selection is made by the individual, the user input 78 sends acontrol signal 84 indicative of the input from the individual to thecontrol module 76. The control module 76 may then send a second controlsignal 86 to the operational mode selector 40 to connect a switch 88 toeither terminal 1 (which places the testing system 10 in regenerativemode) or terminal 2 (which places the testing system 10 in dissipativemode). The control module 76 may refer to, or be part of, an applicationspecific integrated circuit (ASIC), an electronic circuit, acombinational logic circuit, a field programmable gate array (FPGA), aprocessor (shared, dedicated, or group) that executes code, or acombination of some or all of the above, such as in a system-on-chip.

In the non-limiting embodiment as shown, the switch 88 is a single-poledouble-throw switch, however it is to be understood that other types ofswitching elements may be used as well such as, for example, atransistor. As seen in FIG. 1, when a pole 72 of the switch 88 isconnected to terminal 1, then the grid-connected inverter 32 isconnected to the utility line 42, and the testing system 10 is operatingin the regenerative mode. During regenerative mode, energy drawn fromthe EUT 22 may be processed through the ALP 30 (i.e., the VSI 50converts the AC power to DC power), and is sent through the DC link 70to the grid-connected inverter 32. The grid-connector inverter 32converts the DC power from the VSI 50 back into AC power, and then sendsthe AC to the switch 88, and back to the utility line 42. In otherwords, during regenerative mode, the power from the EUT 22 is sent backto the utility line 42 instead of dissipating the power from the EUT 22as heat. Those skilled in the art will readily appreciate that theregenerative mode results in a more efficient, cost-effective testingsystem 10. Moreover, the regenerative mode may also be advantageous insituations where the testing system 10 is located within an environmenthaving limited heat dissipation capabilities.

When the pole 72 of the operational mode selector 40 is connected toterminal 2, then the grid-connected inverter 32 is connected to thedissipative elements 44. In the non-limiting embodiment as shown in FIG.1, the dissipative elements 44 may include a resistor 80 connected inparallel with a capacitor 82, however those skilled in the art willreadily appreciate that any number of elements for dissipatingelectrical energy into heat may be used as well. During dissipativemode, the grid-connected inverter 32 may act as an energy dissipationconverter, where the power from the EUT 22 is transferred through the DClink 70, and is dissipated by the dissipative elements 44. In bothregenerative and dissipative modes, the grid-connected inverter 32 maybe used to regulate the regulated DC voltage source V_(DC), since stableDC voltage is needed by the VSI 50 in order to create specific loadprofiles.

Continuing to refer to FIG. 1, at a given frequency ω, a desired loadcurrent control reference I_(alp) _(_) _(ref)(t) ∠θ_(Ialp) _(_)_(ref)(t) may be determined based on a user-specified or desired loadimpedance Z(t) ∠θ_(z) (t) and a measured terminal voltage V_(EUT)(t)∠θ_(VEUT) (t) across output terminals 68 of the EUT 22 according toEquation 1. Specifically, an individual may enter a specific value ofthe desired load impedance Z (t) ∠θ_(z) (t) by manipulating the seconduser input 79. Equation 1 is expressed as:

$\begin{matrix}{{{I_{{alp}_{ref}}(t)}{{\angle\theta}_{{Ialp}_{ref}}(t)}} = \frac{{V_{EUT}(t)}{{\angle\theta}_{VEUT}(t)}}{z\text{t}{{\angle\theta}_{z}(t)}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

Equation 2 determines an actual load current I_(alp) _(_)(t)∠θ_(Ialp)(t) (the actual load current represents line current throughthe inductor L_(alp)).

$\begin{matrix}{{{I_{alp}(t)}\angle \; {\theta_{Ialp}(t)}} = \frac{{{V_{alp}(t)}\; \angle \; {\theta_{alp}(t)}} - {{V_{EUT}(t)}\; \angle \; {\theta_{VEUT}(t)}}}{r_{alp} + {{j\omega}\; L_{alp}}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

where V_(alp) is the voltage input into the VSI 50, I_(alp) is thecurrent input into the VSI 50, and j is an imaginary component.

Those skilled in the art will readily appreciate that the semiconductors(i.e., the switching elements 54 seen in FIGS. 2A-2C) used in thegrid-connected inverter 32 and the VSI 50 may possess a limited maximumcurrent. Therefore, turning to FIG. 3, a parallel interleaving topologymay be implemented where two or more ALPs 50 that may be substantiallyidentical to one another may be connected in parallel with the EUT 22through respective inductors L_(alp1), L_(alp2), etc. As seen in FIG. 3,the DC link 70 may be used to connect multiple ALPs 50 to multiplegrid-connected inverters 32. It is to be understood that the multiplegrid-connected inverters 32 may also be substantially identical to oneanother, and are also connected in parallel to one another. Thesubstantially identical ALPs 50 and grid-connected inverters 32 enablethe testing system 10 to have a higher rated current, since eachgrid-connected inverter 32 and VSI 50 is limited in power or currentindividually. Those skilled in the art will also appreciate currentsharing control should be used to control the ALPs 50 and grid-connectedinverters 32. This is because the ALPs 50 and grid-connected inverters32 have circuit mismatch, which may lead to unbalanced current sharingif no current sharing mechanism is employed.

FIG. 4A is a detailed schematic diagram of an exemplary programmable ACload 120. The programmable AC load 120 is capable of generatingsteady-state load profiles, lower harmonic load profiles (the exactharmonic orders depend on the specific topology of the system),transient load profiles, and modulation load profiles. As seen in FIG.4A, the programmable AC load 120 includes an active ALP 130 that isbased on a hybrid 5-level VSI 150, an isolated DC link 170, and agrid-connected inverter 132.

The ALP 130 may be based on a hybrid 5-level voltage source invertertopology, however as seen in FIG. 4A, the grid-connected inverter 132may still employ 2-level voltage source inverter topology. Thegrid-connected invertor 132 is connected to the operational modeselector 40 (shown in FIG. 1). In the embodiment as shown, an EUT 122outputs three-phase voltage as V_(a), V_(b), and V_(c) over four voltagelines A, B, C, and N. The ALP 130 is a three-phase hybrid 5-levelvoltage source inverter including three-phase units 164, which are phaseunit A, phase unit B, and phase unit C. Specifically, phase unit A,phase unit B, and phase unit C each include two three-phase legs 160. Asseen in FIG. 4A, each phase leg 160 includes a plurality of powertransistors 154 in combination with antiparallel diodes 156, as well astwo clamping diodes 162. In the exemplary embodiment as shown, thetransistors 154 are IGBTs. In an embodiment, the VSI 150 may becontrolled using sinusoidal pulse width modulation (SPWM) in order toproduce five different voltage levels. It is to be understood that whileFIG. 4A illustrates a three-phase load, the programmable AC load 120 isalso capable of operating as a single phase load as well.

Each phase unit 164 (i.e., phase unit A, phase unit B, and phase unit C)may be connected to three corresponding DC rails 170 a, 170 b, and 170 cthrough corresponding capacitors C1. Specifically, phase unit A isconnected to three DC rails 170 a, phase unit B is connected to three DCrails 170 b, and phase unit C is connected to three DC rails 170 c. Eachof the DC rails 170 a, 170 b, 170 c are connected to at least onebidirectional DC-DC converter 172. Specifically, as seen in FIG. 4A,each phase unit 164 of the VSI 150 is connected to two of the sixbidirectional DC-DC converters 172 through a corresponding set of DCrails 170 a, 170 b, 170 c. The six bidirectional DC-DC converters 172comprise or make up the isolated DC link 170 that connects thegrid-connected inverter 132 with the ALP 130. The six bidirectionalDC-DC converters 172 may be substantially identical to one another. Eachbidirectional DC-DC converter 172 may include the circuitry as seen inFIG. 4B.

Referring to FIG. 4B, in one exemplary embodiment each bidirectionalDC-DC converter 172 may include two full-bridge switching cells 174, ahigh-frequency transformer 180, and a filter inductor L_(f). Eachswitching cell 174 includes a plurality of power transistors 176 (e.g.,IGBTs) in combination with antiparallel diodes 178. Referring to bothFIGS. 4A and 4B, the grid-connected inverter 132 generates a regulatedDC voltage V_(dc) _(_) _(gci) that is sent to the bidirectional DC-DCconverters 172 over a DC link 180. The bidirectional DC-DC converters172 each regulate the DC voltage V_(dc) _(_) _(gci) from thegrid-connected inverter 132 at the same levels, and supply regulated DCvoltage to each of the three-phase units 164 of the ALP 130. As seen inFIG. 4A, the DC link 180 includes only 2 DC rails since thegrid-connected inverter 132 employs 2-level voltage source invertertopology.

Turning back to FIG. 1, it is to be understood that the VSI 50 and thegrid-connected inverter 32 may comprise of semiconductors (i.e., theIGBTs 54 shown in FIGS. 2A-2C) that are generally capable of handlingrelatively high currents. For example, in one embodiment, thesemiconductors are capable of handling current between about 100 toabout 200 Amps. However, these semiconductors may not be able to switchfast enough to produce higher order harmonic load profiles. In theembodiments as described, higher order harmonic load profiles may be anyharmonic that is above fundamental frequency, but is still within systemstability concerns. For example, in one embodiment a higher orderharmonic may be any harmonic from second order to fortieth order.However, those skilled in the art will readily appreciate that thisharmonic range is merely exemplary and a wider range or a narrower rangeof harmonic orders may be used as well. Moreover, the exact range ofharmonic orders depend on the specific topology of the system.

Turning to FIG. 5, an alternative embodiment of a testing system 210capable of generating higher order harmonics is illustrated.Specifically, the testing system 210 may include the ALP 30. The ALP 30includes a standard VSI 50. In addition to the standard VSI 50, thetesting system 210 also includes one or more ALPs 230 that include aplug-in high frequency module 250. The standard VSI 50 includes the sametype of semiconductors as the VSI 50 shown in FIG. 1. In contrast, thehigh frequency module 250 is an inverter including smaller, fastersemiconductor devices capable of generating higher order harmonics. Inparticular, the semiconductors located within the plug-in high frequencymodule 250 include higher switching times (i.e., with a turn-on andturn-off time of less than about 80 ns), thereby resulting in trackinghigher order harmonic current.

For example, in one embodiment, the plug-in high frequency module 250may include IGBTs and MOSFETs having electronic component packaging thatconforms to one of the following transistor outlines: To-220, To-247,To-252, or To-263, which are commonly used standard size semiconductors.These transistor outlines may enable the plug-in high frequency module250 to generate higher order harmonic load profiles, such as secondorder to fortieth order harmonics. As seen in FIG. 5, the plug-in highfrequency module 250 may be connected in parallel with the standard VSI50, and is also connected to an EUT 222 through an inductor L_(alp2).The plug-in high frequency module 250 may also be connected to one ormore grid-connected inverters 32 (shown in FIG. 1) though the DC rail270.

The testing system 210 may also include one or more plug-in transientmodules 252. The plug-in transient module 252 may also be connected inparallel with the standard ALP 30 and the plug-in high frequency module250. The plug-in transient module 252 may be an inverter that is used togenerate non-repeating, transient current profiles. Some examples oftransient current profiles include, but are not limited to, in-rushcurrent transient profiles and short circuit transient profiles.Specifically, the plug-in transient VSI 252 may emulate a transientcondition that occurs during operation of an aircraft. As seen in FIG.5, the plug-in transient module 252 may not be connected to the DC rails270.

FIG. 6 is an illustration of a control structure of the ALP 230 forachieving higher order harmonics. As seen in FIG. 6, a current sensor211 and a voltage sensor 212 may be placed across output terminals 268of the EUT 222. The current sensor 211 senses a real or actual harmonicline current I_(n)∠β at the output terminals 268 of the EUT 222. Theactual harmonic line current I_(n)∠β is composed of fundamentalfrequency components as well as harmonic frequency components. Likewise,the voltage sensor 212 senses a real or actual harmonic line voltageV_(n)∠α. The actual harmonic line voltage V_(n)∠α is also composed offundamental frequency components as well as harmonic components. Theactual harmonic line current and the actual harmonic line voltage are atn^(th) order harmonics, where n=1 at fundamental frequency. Harmonicimpedance may be expressed in Equation 3 as:

$\begin{matrix}{{Z_{n}\angle \; ( {\alpha \text{-}\beta} )},{{{where}\mspace{14mu} Z_{n}} = \frac{V_{n}}{I_{n}}}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

A user input 278 may receive a reference harmonic current I_(nr)∠β_(r)determined by an individual. Specifically, an individual may determine avalue of the reference harmonic current I_(nr)∠β_(r), and inputs thisdesired value into the user input 278. Alternatively, in anotherembodiment, a user may determine a value of a harmonic impedance controlreference Z_(nr)∠γ_(r) instead, and inputs this desired value into theuser input 278. A current control reference calculation block 214determines the reference harmonic current I_(nr)∠β_(r) based on theactual harmonic line voltage V_(n)∠α using Equation 4:

$\begin{matrix}{{I_{nr}\angle \; \beta_{r}} = \frac{V_{n}\angle \; \alpha}{Z_{nr}{\angle\gamma}_{r}}} & {{Equation}\mspace{14mu} 4}\end{matrix}$

The EUT 222 outputs three-phase voltage as V_(a), V_(b), and V_(c). Thecurrent sensor 211 may produce a real-time current measurement I_(abc),which is representative of the current at the output terminals 268 ofthe EUT 222. Likewise, the voltage sensor 212 produces a real-timevoltage measurement V_(abc), which is representative of the voltage atthe output terminals 268 of the EUT 222. As explained in greater detailbelow, the control structure of the programmable AC load 210 may be usedto ensure that the actual harmonic current I_(n)∠β tracks or follows thereference harmonic current I_(nr)∠β_(r), and does not substantiallyimpact the fundamental current control of the ALP 230.

Continuing to refer to FIG. 6, a signal synthesis block 216 may receivethe reference harmonic current I_(nr)∠β_(r). The signal synthesis block216 also receives as input the real-time current measurement I_(abc)from the current sensor 211 as well as the real-time voltage measurementV_(abc) from the voltage sensor 212. FIG. 7 is a detailed block diagramof the signal synthesis block 216. Referring to both FIGS. 6 and 7, athree-phase signal input 228 indicative of the real-time currentmeasurement I_(abc) and the real-time voltage measurement V_(abc)measured across the output terminals 268 of the EUT 222 is inputted intothe signal synthesis block 216.

The signal synthesis block 216 may transfer the three-phase signal input228 into a set of three symmetrical sequences by a sequence decomposer220. Specifically, the sequence decomposer 220 includes a positivesequence decomposition block 280, a negative sequence decompositionblock 282, and a zero sequence decomposition block 284. As explainedbelow, the sequence decomposer 220 transfers the three-phase signalinput 228 into a positive sequence component, a negative sequencecomponent, and a zero sequence component. For example, three voltagephasors V_(A), V_(B) and V_(C) have a positive sequence ABC. Usingsymmetrical sequence components it is possible to represent each phasorvoltage as positive sequence component (i.e., V_(A) ⁺, V_(B) ⁺, V_(C)⁺), a negative sequence component (i.e., V_(A) ⁻, V_(B) ⁻,V_(C) ⁻), anda zero sequence component (i.e., V_(A) ⁰, V_(B) ⁰, V_(C) ⁰).

The positive sequence component rotates in the same direction as asymmetrical ABC sequence. The negative sequence component rotates in anopposite direction of the ABC sequence, and the zero sequence componentis non-rotational. Sequence decomposition may be used for fundamentalfrequencies as well as non-fundamental components (i.e., harmoniccomponents). The harmonic component of the three-phase signal input 228is eventually transferred into an AC signal with a dedicated frequencyin a synchronous rotating domain (dq) time domain, which is alsoexplained in detail below.

The signal synthesis block 216 also includes an abc-to-dq transformer286. Signals generated by the sequence decomposer 220 may be provided asinput to the abc-to-dq transformer 286. The abc-to-dq transformer 286may transform signals from the sequence decomposer 220 into signals thatmay be manipulated in the dq coordinate system. Specifically, theabc-to-dq transformer 286 performs three-phase signal to synchronousrotating frame transformation, where the fundamental three-phase signalinput 228 is represented by two DC quantities, d and q, in the rotatingframe, and the harmonic components are represented by the AC signal witha dedicated frequency in the dq time domain (positive, negative, or zerosequence). Thus, for each sequence component (i.e., the positivesequence component, a negative sequence component, and a zero sequencecomponent), a generalized form for three-phase signals in the dq framemay be expressed by Equation 5. Specifically, an output 288 of thesignal synthesis block 216 may be expressed in Equation 5 as:

C+M _(g1) cos(ω₁ t+φ ₁)+M _(g2) cos(ω₂ t+φ ₂)+M _(g3) cos(ω₃ t+φ ₃)  Equation 5

where C is a dc constant that represents a fundamental component, andthe terms M_(g1) cos (ω₁t+φ₁), M_(g2) cos(ω₂t+φ₂), etc. representharmonic components.

Referring to both FIGS. 6 and 7, a control module 218 may be inoperative communication to receive an error signal E from the signalsynthesis block 216. The signal synthesis block 216 may include an errorsignal block 290 for determining the error signal E. As explained ingreater detail below, the error signal E may be based on the differencebetween the reference harmonic current I_(nr)∠β_(r) and the output 288determined by the signal synthesis block 216.

FIG. 8 is an exemplary illustration of the error signal block 290. Theerror signal block 290 may include a abc-to-dq transformer 300 thatreceives the reference harmonic current I_(nr)∠β_(r) from the currentcontrol reference calculation block 214 (shown in FIG. 6). The abc-to-dqtransformer 300 may transform the reference harmonic currentI_(nr)∠β_(r) into signals that may be manipulated in the dq coordinatesystem. In particular, the abc-to-dq transformer 300 determines acontrol reference current value i_(dqr) that is expressed in the dqcoordinate system. The control reference current value i_(dqr) may besent to an error calculation block 302. The output 288 determined by thesignal synthesis block 218, which is already expressed in the dq frame,may also be sent the error calculation block 302. The error calculationblock 302 may subtract the control reference current value i_(dqr) fromthe output 288 in order to obtain the error signal E.

Turning back to FIG. 6, the control module 218 may be in operativecommunication with a single plug-in high frequency module 250. Thecontrol module 218 outputs a control duty cycle signal d_(dq) based onthe error signal E. The duty cycle signal d_(dq) is first transformedfrom the dq coordinate system back to the abc coordinate system by adq-to-abc transformer block 304. The transformed duty cycle signald_(abc) may then be sent to the plug-in high frequency module 250. Thetransformed duty cycle signal d_(abc) indicates the switching frequencyfor the semiconductors contained within the corresponding plug-in highfrequency module 250.

The control module 218 may be based on classic dq decoupling controlstructure. Classic dq decoupling control structure occurs whenthree-phase fundamental AC signals are transformed into the synchronousrotating domain dq for DC quantities. The control module 218 may bedesigned to handle multiple dq quantities at different sequences. Inother words, the positive sequence component, the negative sequencecomponent, and the zero sequence component generated by sequencedecomposer 220 (FIG. 7) may each generate dq components. A transferfunction P(s) of the plant (i.e., the programmable AC load 210) may beexpressed in Equation 6 as:

$\begin{matrix}{{P(s)} = {\frac{i_{dq}}{d_{dq}} = \frac{V_{DC}}{{Ls} + r_{L}}}} & {{Equation}\mspace{14mu} 6}\end{matrix}$

where i_(dq) is the real-time current measurement, V_(DC) is theregulated DC voltage source from the grid-connected inverter 232 (FIG.5), d_(dq) is the control duty cycle, L is the inductance from theinductor L_(alp), and r_(L) is the equivalent serial resistance of theinductor L_(alp).

In order to track the dc constant C of the output of the signalsynthesis block 216 expressed in Equation 5, the control module 218 mayinclude design parameters based on Equation 7:

$\begin{matrix}{C_{d} = \frac{K}{s}} & {{Equation}\mspace{14mu} 7}\end{matrix}$

where C_(d) represents the control module 218, K is gain, and srepresents a parameter in the Laplace domain. It is to be understoodthat the gain K is selected such that the system may track the controlreference current value i_(dqr) within a specific tolerance range (i.e.,+/−2%).

The tracking of the harmonic components of the output of the signalsynthesis block 216 expressed in Equation 5, (i.e., the AC signalcomponents) may now be described. For purposes of simplicity, the outputof the signal synthesis block 216 may be expressed using only onesinusoidal term (which corresponds to a dedicated harmonic) with afrequency of ω, and is shown in Equation 8 as:

C+M _(g) cos(ωt+φ)   Equation 8

It is to be understood that although Equation 8 only expresses a singlefrequency ω, the control module 218 performs multiple calculations inparallel with one another in order to determine the various harmoniccomponents included within the output of the signal synthesis block 216(Equation 5). Equation 9 is an expression of a closed-loop controlreference to output transfer function, which is used to track thefrequency ω expressed in Equation 8 (where the frequency ω is asinusoidal component). Specifically, Equation 9 may be expressed as:

$\begin{matrix}{\frac{i_{dq}}{i_{dqr}} = {\frac{2\; \omega_{c}s}{s^{2} + {2\; \omega_{c}s} + \omega^{2}} = \frac{M\frac{V_{DC}}{L_{S} + r_{L}}}{1 + {M\frac{V_{DC}}{L_{S} + r_{L}}}}}} & {{Equation}\mspace{14mu} 9}\end{matrix}$

where ω_(c) is the pass band bandwidth, and M represents the controlmodule 218. Solving for M in Equation 9 yields Equation 10:

$\begin{matrix}{M = \frac{2\; \omega_{c}{s( \frac{L_{S} + r_{L}}{V_{DC}} )}}{s^{2} + \omega^{2}}} & {{Equation}\mspace{14mu} 10}\end{matrix}$

The control module 218 has zero dc gain. The control module 218calculates each of the various harmonic components included within theoutput of the signal synthesis block 216 in parallel with another, andalso calculates the dc component C of the output of the signal synthesisblock 216 in parallel as well. However, since there is zero DC gain,tracking the dc component C may not be affected by the calculation ofthe various harmonic components. In particular, a dc quantity controlloop crossover frequency, which represents a parameter of DC quantitycontrol, may be relatively low when compared to the angular frequency ω.Thus, those skilled in the art will readily appreciate that the controlmodule 218 may be designed to determine multiple harmonic frequencies,without affecting DC quantity control, which corresponds to thefundamental component control of the ALP 230.

FIG. 9 is an alternative embodiment of a testing system 310, where anALP 330 may provide current modulation load profiles. The ALP 330 mayinclude a VSI 350 that is connected to a grid-connected inverter (shownin FIG. 1). It is to be understood that unlike the VSI 50 shown in FIG.6, the VSI 350 may not require semiconductors having relatively fastswitching times (i.e., IGBTs and MOSFETs having To-220, To-247, To-252,or To-263 transistor outlines).

Continuing to refer to FIG. 9, a current sensor 311 and a voltage sensor312 may be placed across output terminals 368 of the EUT 222. Thecurrent sensor 311 may produce a real-time current measurement I_(abc),which is representative of the current at the output terminals 368 ofthe EUT 322. Likewise, the voltage sensor 312 produces a real-timevoltage measurement V_(abc), which is representative of the currentacross the output terminals 368 of the EUT 322.

The testing system 310 may also include a user input 370, a voltageabc-to-dq transformer 380, a current abc-to-dq transformer 382, areference calculation block 384, an error signal block 386, a controlmodule 318, and a abc-to-dq transformer 334. The voltage abc-to-dqtransformer 380 may be used to transform the 3-phase real-time voltagemeasurement V_(abc) detected by the voltage sensor 312 into a real-timevoltage signal V_(dq) that may be manipulated in the dq coordinatesystem. Similarly, the current abc-to-dq transformer 382 may be used totransform the 3-phase real-time current measurement I_(abc) detected bythe voltage sensor 311 into a real-time current signal i_(dq) that maybe manipulated in the dq coordinate system.

The reference calculation block 384 receives as input the real-timevoltage signal V_(dq) from the voltage abc-to-dq transformer 380 as wellas a three-phase balanced current control reference I_(abcr) valuereceived from the user input 370. As explained in detail below, thereference calculation block 384 determines the control reference currentvalue i_(dqr) based on the real-time voltage signal V_(dq) and thethree-phase balanced current control reference I_(abcr).

The calculation of the control reference current value i_(dqr) by thereference calculation block 384 may now be explained. The three-phasebalanced current control reference I_(abcr) may be expressed in vectorform in Equation 11 as:

$\begin{matrix}{i_{dqr} = {\begin{bmatrix}i_{ar} \\i_{br} \\i_{cr}\end{bmatrix} = \begin{bmatrix}{I_{p}{\cos ( {{\omega \; t} + \theta} )}} \\{I_{p}{\cos ( {{\omega \; t} + \theta - \frac{2\; \pi}{3}} )}} \\{I_{p}{\cos ( {{\omega \; t} + \theta + \frac{2\pi}{3}} )}}\end{bmatrix}}} & {{Equation}\mspace{14mu} 11}\end{matrix}$

where I_(p) is the magnitude of a three-phase balanced current controlreference I_(abcr) and is a constant, θ is phase, and ω is angularfrequency (where ω=2πf, and f is frequency). Equation 12 is theabc-to-dq0 transformation matrix that is used to transform abcquantities into dq0 quantities. In Equation 13, the correspondingcontrol reference in dq0 domain is obtained. In particular, Equation 13expresses that given a three-phase current control reference I_(abcr)that is balanced and fixed, the corresponding d and q quantities areconstant, depending on an initial phase θ and magnitude I_(p).

$\begin{matrix}{T = {\sqrt{\frac{2}{3}}\begin{bmatrix}{\cos \; \omega \; t} & {\cos ( {{\omega \; t} - \frac{2\; \pi}{3}} )} & {\cos \; ( {{\omega \; t} + \frac{2\; \pi}{3}} )} \\{{- \sin}\; \omega \; t} & {- {\sin ( {{\omega \; t} - \frac{2\; \pi}{3}} )}} & {{- \sin}\; ( {{\omega \; t} + \frac{2\; \pi}{3}} )} \\\frac{1}{\sqrt{2}} & \frac{1}{\sqrt{2}} & \frac{1}{\sqrt{2}}\end{bmatrix}}} & {{Equation}\mspace{14mu} 12} \\{i_{{dq}\; 0\; r} = {\begin{bmatrix}i_{dr} \\i_{dr} \\i_{0\; r}\end{bmatrix} = {{Ti}_{abcr} = {\frac{\sqrt{6}}{2}{\begin{matrix}{I_{p}\cos \; \theta} \\{I_{p}\sin \; \theta} \\0\end{matrix}}}}}} & {{Equation}\mspace{14mu} 13}\end{matrix}$

Equation 14 expresses current magnitude modulation based on thethree-phase balanced current control reference I_(abcr). As seen inEquation 14, the magnitude of the three-phase balanced current controlreference I_(abcr) is no longer constant. Instead, a modulation termi_(m) cos(ω_(m)t+θ_(m)) exists, where i_(m) represents the modulationmagnitude.

$\begin{matrix}{i_{dqr} = {\begin{bmatrix}i_{ar} \\i_{br} \\i_{cr}\end{bmatrix} = \begin{bmatrix}{( {I_{p} + {i_{m}\cos \; ( {{\omega_{m}t} + \theta_{m}} )}} )\cos \; ( {{\omega \; t} + \theta} )} \\{( {I_{p} + {i_{m}{\cos ( {{\omega_{m}t} + \theta_{m}} )}}} )\cos \; ( {{\omega \; t} + \theta - \frac{2\; \pi}{3}} )} \\{( {I_{p} + {i_{m}{\cos ( {{\omega_{m}t} + \theta_{m}} )}}} ){\cos ( {{\omega \; t} + \theta + \frac{2\; \pi}{3}} )}}\end{bmatrix}}} & {{Equation}\mspace{14mu} 14}\end{matrix}$

Applying abc-to-dq transformation to Equation 14 yields Equation 15,which is:

$\begin{matrix}{i_{dqr} = {\begin{bmatrix}i_{ar} \\i_{br} \\i_{0r}\end{bmatrix} = {{Ti}_{abc} = {\frac{\sqrt{6}}{2}\begin{bmatrix}{( {{I_{p}\cos \; \theta} + {i_{m}{\cos ( {{\omega_{m}t} + \theta_{m}} )}}} )\cos \; \theta} \\{( {{I_{p}\sin \; \theta} + {i_{m}{\cos ( {{\omega_{m}t} + \theta_{m}} )}}} )\sin \; \theta} \\0\end{bmatrix}}}}} & {{Equation}\mspace{14mu} 15}\end{matrix}$

As seen in Equation 15, the d and the q quantities (i_(dr) and i_(qr))are no longer constant, and instead include a dc quantity that isconstant, plus a sinusoidal modulation term.

It is to be understood that Equations 11-15 are expressed as only singlemodulation frequencies. Equations 16 and 17 express Equation 15 inmultiple modulation frequencies (i.e., ω_(m1), ω_(m2), ω_(m3), . . . )with a given modulation magnitude (i.e., i_(m1), i_(m2), i_(m3), . . . )and corresponding initial phase (θ_(m1), θ_(m2), θ_(m3), . . . ):

$\begin{matrix}{i_{dq0r} = {\begin{bmatrix}i_{ar} \\i_{br} \\i_{0r}\end{bmatrix} = {T_{iabc} = {\frac{\sqrt{6}}{2}{A\begin{bmatrix}{\cos \; \theta} \\{\sin \; \theta} \\0\end{bmatrix}}}}}} & {{Equation}\mspace{14mu} 16}\end{matrix}$wherein A=I _(p) +i _(m1) cos(ω_(m1) t+θ _(m1))+i _(m2) cos(ω_(m2) t+θ_(m2))+. . . i _(mn) cos(ω_(mn) t+θ _(mn))   Equation 17

For purposes of simplicity, Equations 16 and 17 may be expressed usingonly one modulation frequency, i_(m) cos(ω_(m)t+θ_(m)), and thecorresponding current dq domain control reference may be expressed inEquation 18 as:

$\begin{matrix}{i_{dqr} = {\begin{bmatrix}i_{dr} \\i_{qr}\end{bmatrix} = {T_{iabc} = {\frac{\sqrt{6}}{2}\begin{bmatrix}{( {{I_{p}\cos \; \theta} + {i_{m}{\cos ( {{\omega_{m}t} + \theta_{m}} )}}} )\cos \; \theta} \\{( {{I_{p}\sin \; \theta} + {i_{m}{\cos ( {{\omega_{m}t} + \theta_{m}} )}}} )\sin \; \theta}\end{bmatrix}}}}} & {{Equation}\mspace{14mu} 18}\end{matrix}$

Continuing to refer to FIG. 9, the control reference current valuei_(dqr) may be sent from the reference calculation block 384 to theerror signal block 386. The error signal block 386 may calculate theerror signal E. Similar to the embodiment as shown in FIG. 6, the errorsignal E is based on the difference between the real-time currentmeasurement i_(dq) measured by the current sensor 311 and the controlreference current value i_(dqr). The error signal E may be sent to thecontrol module 318.

The control module 318 includes similar logic as the control module 218shown in FIG. 6. Specifically, similar to the embodiment as shown inFIG. 6, the control module 318 outputs the control duty cycle signald_(dq) based on the error signal E. The duty cycle signal d_(dq) isfirst transformed from the dq coordinate system back to the abccoordinate system by a dq-to-abc transformer block 334. The transformedduty cycle signal d_(abc) is then sent to the VSI 350. The transformedduty cycle signal d_(abc) indicates the switching frequency and dutycycle for the semiconductors contained within the VSI 350.

Moreover, similar to the control module 218 the control module 310 alsohas zero dc gain. Thus, the tracking the dc component C may not beaffected by determining current modulation load profiles. In particular,the dc quantity control loop crossover frequency may be relatively lowwhen compared to the angular frequency ω. Thus, those skilled in the artwill readily appreciate that the control module 318 may be combined withmultiple resonant control modules in parallel without affecting DCquantity control, which corresponds to the fundamental component controlof the ALP 330.

Referring generally to the figures, the disclosed programmable AC loadprovides several advantages and benefits when compared to some of theprogrammable AC loads currently available. First, the disclosedprogrammable AC load includes a regenerative mode, which allows for thepower from the EUT to be sent back to the utility line instead ofdissipating the power from the EUT as heat. Those skilled in the artwill readily appreciate that the regenerative mode results in a moreefficient, cost-effective testing system. Moreover, the disclosedprogrammable AC load may also be more suitable for testing environmentswhich have limited heat dissipation capabilities.

In addition to the regenerative mode, the disclosed ALP programmable ACload may utilize an active power electronic circuit, rather than usingcombinations of passive elements. The programmable AC load may provide avariety of load profile functions to the EUT such as steady-state loadprofiles, lower harmonic load profiles (the exact harmonic orders dependon the specific topology of the system), transient load profiles,modulation load profiles, higher order harmonic load profiles, andcurrent modulation load profiles. In particular, the disclosedprogrammable AC load may generate higher order harmonic profiles, whereharmonic magnitude and phase may be selected by a user without the needto change any hardware within the testing system. Moreover, multipleharmonics may also be generated. The programmable AC load may alsogenerate higher order harmonic profiles using an approach that does notaffect the dc component in the dq domain. Finally, the disclosedprogrammable AC load may also generate current modulation load profilesas well, without modifying hardware within the testing system.

While the forms of apparatus and methods herein described constitutepreferred aspects of this disclosure, it is to be understood that thedisclosure is not limited to these precise forms of apparatus andmethods, and the changes may be made therein without departing from thescope of the disclosure.

What is claimed is:
 1. A programmable alternating current (AC) load incommunication with an equipment under test (EUT), the EUT generating anequipment under test voltage, the programmable AC load comprising: anactive load profiler (ALP) creating a specific load profile sent to theEUT, the ALP including a voltage source inverter having an AC side and adirect current (DC) side, the AC side of the voltage source inverter incommunication with the EUT for receiving the equipment under testvoltage; a grid-connected inverter having an AC side and a DC side, theDC side of the grid-connected inverter in communication with the DC sideof the voltage source inverter of the ALP, the grid-connected inverterproviding a regulated DC voltage source to the voltage source inverterof the ALP; and an operational mode selector in communication with theAC side of the grid-connected inverter, the operational mode selectorplacing the programmable AC load in either a regenerative mode where theequipment under test voltage is sent to a main grid or a dissipativemode where the equipment under test voltage is dissipated by heat. 2.The programmable AC load of claim 1, wherein the operational modeselector comprises a switching element that places the programmable ACload in either the regenerative mode or the dissipative mode.
 3. Theprogrammable AC load of claim 1, comprising a series of dissipativeelements that are connected to the grid-connected inverter in thedissipative mode.
 4. The programmable AC load of claim 1, comprising auser input in communication with a control module, the control module incommunication with the operational mode selector.
 5. The programmable ACload of claim 4, wherein the user input sends a control signal to thecontrol module indicative of a user-generated input, wherein theuser-generated input indicates whether the programmable AC load is inregenerative mode or dissipative mode.
 6. The programmable AC load ofclaim 1, wherein the voltage source inverter is selected from the groupconsisting of: a single phase, 2-level inverter, a three-phase 2-levelinverter, and a three-phase cascade 3-level inverter.
 7. Theprogrammable AC load of claim 1, comprising a DC link connecting the DCside of the voltage source inverter to the DC side of the grid-connectedinverter.
 8. The programmable AC load of claim 1, wherein the ALPcomprises at least one plug-in high frequency module connected inparallel with the ALP, the at least one plug-in high frequency modulegenerating higher order harmonic load profiles.
 9. The programmable ACload of claim 8, wherein the at least one plug-in high frequency modulecomprises a plurality of semiconductors including electronic componentpackaging that conforms to transistor outlines selected from the groupconsisting of: To-220, To-247, To-252, and To-263.
 10. The programmableAC load of claim 8, wherein the higher order harmonic load profilesrange from a second order harmonic to a fortieth order harmonic.
 11. Theprogrammable AC load of claim 1, wherein the ALP comprises at least oneplug-in transient module connected in parallel with the voltage sourceinverter, the at least one plug-in transient module generating anon-repeating transient current profile.
 12. The programmable AC load ofclaim 11, wherein the non-repeating transient current profiles includein-rush current transient profiles and short circuit transient profiles.13. The programmable AC load of claim 1, wherein the ALP comprises aninductor located between the AC side of the voltage source inverter andthe EUT.
 14. The programmable AC load of claim 1, wherein the ALP isbased on a hybrid 5-level voltage source inverter topology and thegrid-connected inverter is based on 2-level voltage source invertertopology.
 15. A programmable alternating current (AC) load incommunication with an equipment under test (EUT), the EUT generating anequipment under test voltage, the programmable AC load comprising: anactive load profiler (ALP) creating a specific load profile sent to theEUT, the ALP comprising: a voltage source inverter having an AC side anda direct current (DC) side, the AC side of the voltage source inverterin communication with the EUT for receiving the equipment under testvoltage; at least one plug-in high frequency module connected inparallel with the ALP and generating higher order harmonic loadprofiles; and a control module in operative communication with acorresponding plug-in high frequency module, the control module sendinga control duty signal to the corresponding plug-in high frequency moduleindicating a switching frequency; a grid-connected inverter having an ACside and a DC side, the DC side of the grid-connected inverter incommunication with the DC side of the voltage source inverter of theALP, the grid-connected inverter providing a regulated DC voltage sourceto the voltage source inverter of the ALP; and an operational modeselector in communication with the AC side of the grid-connectedinverter, the operational mode selector placing the programmable AC loadin either a regenerative mode where the equipment under test voltage issent to a main grid or a dissipative mode where the equipment under testvoltage is dissipated by heat.
 16. The programmable AC load of claim 15,comprising a signal synthesis block in communication with the controlmodule, the signal synthesis block receiving as input a referenceharmonic current, a real-time current measurement measured at outputterminals of the EUT and a real-time voltage measurement measured atoutput terminals of the EUT.
 17. The programmable AC load of claim 16,wherein the signal synthesis block transforms the real-time currentmeasurement and the real-time voltage measurement into a positivesequence component, a negative sequence component, and a zero sequencecomponent.
 18. The programmable AC load of claim 16, wherein the signalsynthesis block determines an output expressed as:C+M _(g1) cos(ω₁ t+φ ₁)+M _(g2) cos(ω₂ t+φ ₂)+M _(g3) cos(ω₃ t+φ ₃)wherein C is a dc constant that represents a fundamental component, andwherein the terms M_(g1) cos(ω₁t+φ₁), M_(g2) cos(ω₂t+φ₂), M_(g3)cos(ω₃t+φ₃) represent harmonic components.
 19. The programmable AC loadof claim 18, wherein the signal synthesis block includes an error signalblock that determines an error signal that is sent to the controlmodule.
 20. The programmable AC load of claim 19, wherein the errorsignal is determined based on a difference between the referenceharmonic current and the output determined by the signal synthesisblock.
 21. The programmable AC load of claim 15, comprising a user inputin communication with a control module, the control module incommunication with the operational mode selector.
 22. The programmableAC load of claim 21, wherein the user input sends a control signal tothe control module indicative of a user-generated input, wherein theuser-generated input indicates whether the programmable AC load is inregenerative mode or dissipative mode.
 23. The programmable AC load ofclaim 15, wherein a plurality of semiconductors within the at least oneplug-in high frequency module include electronic component packagingthat conforms to transistor outlines selected from the group consistingof: To-220, To-247, To-252, and To-263.
 24. A programmable alternatingcurrent (AC) load in communication with an equipment under test (EUT),the EUT generating an equipment under test voltage, the programmable ACload comprising: an active load profiler (ALP) creating a specific loadprofile sent to the EUT, the ALP comprising: a voltage source inverterhaving an AC side and a direct current (DC) side, the AC side of thevoltage source inverter in communication with the EUT for receiving theequipment under test voltage; a control module in operativecommunication with the voltage source inverter of the ALP, the controlmodule sending a control duty signal to the voltage source inverterindicating a switching frequency and a duty cycle; a grid-connectedinverter having an AC side and a DC side, the DC side of thegrid-connected inverter in communication with the DC side of the voltagesource inverter of the ALP, the grid-connected inverter providing aregulated DC voltage source to the voltage source inverter of the ALP;and an operational mode selector in communication with the AC side ofthe grid-connected inverter, the operational mode selector placing theprogrammable AC load in either a regenerative mode where the equipmentunder test voltage is sent to a main grid or a dissipative mode wherethe equipment under test voltage is dissipated by heat.
 25. Theprogrammable AC load of claim 24, comprising a voltage sensor placedacross output terminals of the EUT for producing a real-time voltagemeasurement.
 26. The programmable AC load of claim 25, comprising areference calculation block receiving as input the real-time voltagemeasurement from the voltage sensor.
 27. The programmable AC load ofclaim 26, wherein the reference calculation block receives a three-phasebalanced current control reference value, wherein the referencecalculation block determines a control reference current value based onthe real-time voltage measurement and the three-phase balanced currentcontrol reference value.
 28. The programmable AC load of claim 27,comprising an error signal block that receives the control referencecurrent value from the reference calculation block, wherein the errorsignal block determines an error signal based on a difference between areal-time current measurement and the control reference current value,and wherein the error signal is sent to the control module.
 29. Theprogrammable AC load of claim 24, wherein the control module has zero dcgain.
 30. A method of creating a specific load profile by a programmablealternating current (AC) load that is sent to an equipment under test(EUT), the EUT generating an equipment under test voltage, the methodcomprising: creating a specific load profile sent to the EUT by anactive load profiler (ALP), the ALP including a voltage source inverter;providing a regulated direct current (DC) voltage source to the voltagesource inverter by a grid-connected inverter; and placing theprogrammable AC load in either a regenerative mode where the equipmentunder test voltage is sent to a main grid or a dissipative mode wherethe equipment under test voltage is dissipated by heat by an operationalmode selector in communication with the grid-connected inverter.
 31. Themethod of claim 30, comprising providing a user input in communicationwith a control module, the control module in communication with theoperational mode selector.
 32. The method of claim 31, comprisingsending a control signal by the user input to the control moduleindicative of a user-generated input, wherein the user-generated inputindicates whether the programmable AC load is in regenerative mode ordissipative mode.